System and Method for a Transducer Interface

ABSTRACT

According to an embodiment, an interface circuit includes a current replicator and a receiver. The current replicator includes a power terminal coupled to a first reference node, an output terminal configured to output a signal proportional to a signal received from a transducer, and an interface terminal coupled to the transducer. Using a single interface terminal, the current replicator may be configured to provide power to the transducer and receive output signals from the transducer. The receiver may include a first input terminal coupled to the output terminal, a second input terminal coupled to a second reference node, and a current converter circuit coupled to the first input terminal.

This is a Continuation application of U.S. application Ser. No.14/060,391, entitled “System and Method for a Transducer Interface”which was filed on Oct. 22, 2013 and is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to transducer systems, and, inparticular embodiments, to a system and method for a transducerinterface.

BACKGROUND

Audio microphones are commonly used in a variety of consumerapplications such as cellular telephones, digital audio recorders,personal computers and teleconferencing systems. In particular,lower-cost electret condenser microphones (ECM) are used in massproduced cost sensitive applications. An ECM microphone typicallyincludes a film of electret material that is mounted in a small packagehaving a sound port and electrical output terminals. The electretmaterial is adhered to a diaphragm or makes up the diaphragm itself.

Another type of microphone is a microelectromechanical systems (MEMS)microphone, in which a pressure sensitive diaphragm is etched directlyonto an integrated circuit. As such, the microphone is contained on asingle integrated circuit rather than being fabricated from individualdiscrete parts.

Most ECM and MEMS microphones also include a preamplifier that can beinterfaced to an audio front-end amplifier via a cord and plug for atarget application such as a cell phone or a hearing aid. In many cases,the interface between the preamplifier and front-end amplifier is athree-wire interface coupled to a power terminal, signal terminal andground terminal. In some systems, however, a two-wire interface is usedin which two of the terminals are combined into a signal, therebyreducing the cost of the system by using two wires instead of threewires.

For all types of transducers, and microphones in particular, designingelectronic interfaces can be challenging. Combining a power and signalinterface into a single interface poses a number of particular designchallenges with respect to voltage swing and low supply voltage use,among others. Increasing voltage swing in a transducer generallyincreases the range of the transducer. Lowering the supply voltage isusually associated with decreasing the power consumption and is oftenrelevant in mobile applications. In some instances, lowering the supplyvoltage, however, may have an adverse impact on the voltage swing.

SUMMARY OF THE INVENTION

According to an embodiment, an interface circuit includes a currentreplicator and a receiver. The current replicator includes a powerterminal coupled to a first reference node, an output terminalconfigured to output a signal proportional to a signal received from atransducer, and an interface terminal coupled to the transducer. Using asingle interface terminal, the current replicator may be configured toprovide power to the transducer and receive output signals from thetransducer. The receiver may include a first input terminal coupled tothe output terminal, a second input terminal coupled to a secondreference node, and a current converter circuit coupled to the firstinput terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a block diagram of an embodiment microphone system;

FIG. 2 illustrates a block diagram of an embodiment transducer system;

FIG. 3 illustrates a schematic of an embodiment current replicator;

FIG. 4 illustrates a schematic of another embodiment current replicator;

FIG. 5 illustrates a block diagram of another embodiment microphonesystem;

FIG. 6 illustrates a block diagram of a further embodiment microphonesystem; and

FIG. 7 illustrates a block diagram of an embodiment method of operation.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Description is made with respect to various embodiments in a specificcontext, namely microphone transducers, and more particularly,microphone interface circuits. Some of the various embodiments describedherein include transducer systems, microphone systems, interfacecircuits for transducer and MEMS transducer systems, and two wire andthree wire transducer interfaces. In other embodiments, aspects may alsobe applied to other applications involving any type of sensor ortransducer interfacing with electronics according to any fashion asknown in the art.

According to various embodiments described herein, a transducer systemincludes a current replicator coupled to a transducer and a codec. Thecurrent replicator supplies power and receives transduced signals on asingle coupling between the current replicator and the transducer. Thecurrent replicator also supplies a replicated current signal,proportional to the transduced signal, to a current converter circuitcoupled to the codec. In some embodiments, the current converter circuitconverts the replicated current signal to a voltage signal and suppliesthe voltage signal to the codec.

FIG. 1 illustrates a block diagram of an embodiment microphone system100 including a microphone 120, a current replicator 110, and a codec130. In various embodiments, the microphone 120 receives acousticsignals and transduces the acoustic signals into electrical signals tosupply to current replicator 110. Current replicator 110 produces areplica current to provide to codec 130 via impedance 140. In theembodiment shown in FIG. 1, only two wires are coupled betweenmicrophone 120 and a remainder of the system.

According to various embodiments, microphone 120 includes an acoustictransducing element 122 which may include a MEMS transducer element witha backplate and a membrane. In various embodiments, the acoustictransducing element 122 may include numerous transducer elements, MEMStransducer elements, and other transducers and MEMS transducers.Acoustic transducing element 122 may be made using microfabricationtechniques and may be fabricated on a single die or on multiple dies.

In the embodiment shown in FIG. 1, acoustic transducing element 122 iscoupled to an input of amplifier 124. Amplifier 124 is coupled to apower terminal VDD, an output terminal OUT, and a reference terminalVSS. Output terminal OUT may be coupled to reference terminal VSS viaimpedance 126, thereby making microphone 120 function as a two wiredevice. In such an embodiment, amplifier 124 drives output terminal OUTwith a transduced voltage signal. The voltage signal may drive a currentthrough impedance 126. Fluctuations in the current consumption at powerterminal VDD are produced due to the current flowing through impedance126. Such fluctuations can serve as an output signal at power terminalVDD proportional to a transduced signal from acoustic transducingelement 122. In such embodiments, microphone 120 only uses referenceterminal GND and power terminal VDD configured to sense fluctuations incurrent draw.

In some embodiments, microphone 120 may be implemented, for example,using circuitry described in U.S. patent application Ser. No.13/941,273, filed on Jul. 12, 2013 and entitled “System and Method for aMicrophone Amplifier,” which is incorporated herein by reference in itsentirety.

According to various embodiments, current replicator 110 is configuredto sense fluctuations in current draw through power terminal VDD ofmicrophone 120. As shown, power terminal VDD is coupled to terminal 1 ofcurrent replicator 110. Terminal 2 is coupled to codec inputs INP andINN and terminal 3 is coupled to supply voltage VDD_ext. In someembodiments, current replicator 110 is configured to maintain a constantor substantially constant voltage on terminal 1 while the current drawfluctuates. In an embodiment, current replicator 110 is configured toproduces a replica current at terminal 2 that is equal or proportionalto the current drawn from terminal 1. In a particular embodiment, a verysmall voltage drop is present between terminal 3 and terminal 1 ofcurrent replicator 110. A voltage at terminal 2 may swing between groundand a very small voltage drop below supply voltage VDD_ext. In someembodiments, current replicator 110 enables a large signal swing whilemaintaining a supply voltage to power terminal VDD via terminal 1.

In the embodiment shown in FIG. 1, the replicated current at terminal 2is produced by current replicator 110 and flows through impedance 140.Codec 130 receives a voltage signal input from inputs INP and INN. Codecinputs INP and INN provide the voltage drop across impedance 140. Insome embodiments, input INN may be coupled to ground GND, as shown.Codec 130 encodes the voltage signal input that is proportional to anoriginal transduced signal from acoustic transducing element 122.Encoding may include amplifying and performing analog to digitalconversion. As such, codec 130 may include an amplifier and an analog todigital converter (ADC).

FIG. 2 illustrates a block diagram of an embodiment transducer system200 including a transducer 220, a current replicator 210, and a codecinterface. Transducer 220 is shown as a circuit model of a generictransducer comprising a variable current source 223. Coupled in parallelto the current source 223 are transducer resistance 225 and DC currentsource 227. In various embodiments, transducer 220 may include any typeof transducer as is known in the art. In a specific embodiment,transducer 220 is a MEMS microphone.

As shown, current replicator 210 is coupled to transducer power terminalVDD and is configured to measure a current I_(MIC) at power terminalVDD. In various embodiments, current replicator 210 may produce areplica current I_(MIC) _(_) _(C) proportional to, and in some casesequal to, current I_(MIC). Replica current I_(MIC) _(_) _(C) may flowthrough resistor 240 in order to produce a voltage signal input at thecodec input. In the embodiment shown, the codec interface is modeled asa resistive and capacitive interface by resistor 232 and capacitors 234and 235.

FIG. 3 illustrates a schematic of an embodiment current replicator 300including a first transistor 302 and a second transistor 304 configuredas a current mirror. In various embodiments, transistors 302 and 304 arecoupled to supply voltage VDD_EXT through terminal 3 and controlterminals of transistors 302 and 304 may be coupled together. Transistor302 may be configured to supply current I_(MIC) to terminal 1 in orderto maintain a substantially constant voltage on terminal 1. As shown,the control terminal of transistor 302 may be coupled to an output oftransistor 302 at terminal 1. In various embodiments, transistor 304 mayproduce a replica current I_(MIC) _(_) _(C) proportional to currentI_(MIC). In some embodiments, the proportionality is given by the ratiobetween the sizes of transistors 302 and 304.

FIG. 4 illustrates a schematic of another embodiment current replicator400 including transistors 402 and 404, amplifier 406, and referencevoltage source 408. In various embodiments, current replicator 400 isconfigured to maintain a substantially constant voltage on terminal 2 byvarying the current I_(MIC) flowing through transistor 404 whileproducing a proportional replica current I_(MIC) _(_) _(C) throughtransistor 402.

In the embodiment shown, transistor 402, transistor 404, and amplifier406 have supply terminals coupled to supply voltage VDD_EXT via terminal3. An output of amplifier 406 supplies control terminals of bothtransistors 402 and 404, which control terminals are coupled together.In such embodiments, replica current I_(MIC) _(_) _(C) is proportionalto I_(MIC) because both transistors 402 and 404 have the same supplyvoltage VDD_EXT and receive a same control signal from amplifier 406. Asshown, transistor 404, amplifier 406, and reference voltage source 408may together form a low-dropout regulator LDO.

According to various embodiments, amplifier 406 has a reference terminalcoupled to a reference voltage, which is shown as a ground connection.Amplifier 406 may also have a first input coupled to reference voltagesource 408 having a voltage Vref and a second input coupled to an outputof transistor 404 at terminal 2. In various embodiments, voltage Vrefmay take on any value for biasing amplifier 406. In a particularembodiment, amplifier 406 functions to control transistor 404 in orderto maintain a constant voltage on terminal 2. Specifically, amplifier406 may function as a feedback amplifier for transistor 404 whilesimultaneously producing a proportional current in transistor 402.

In the embodiment shown in FIG. 4, amplifier 406 is depicted as anoperational amplifier and transistors 402 and 404 are depicted asMOSFETs. In other embodiments, amplifier 406 may be implemented as anytype of amplifier and may include multiple stages. Further, inalternative embodiments, transistors 402 and 404 may be implemented asany type of transistor and may include multiple transistors of a samesize or multiple transistors with various different sizes. In thiscontext, transistor size primarily refers to gate length and width of atransistor, but may also include other dimensions within a transistor.In reference to FIG. 3, numerous modifications to circuit arrangementand transistor types or sizes are also possible as is known in the art.

FIG. 5 illustrates a block diagram of another embodiment microphonesystem 500 including a microphone 520, a current replicator 510, and acodec 530, as described previously. As shown, in microphone system 500codec 530 includes current replicator 510, impedance 540, and anamplifier 532. According to various embodiments, codec 530, currentreplicator 510, impedance 540, and amplifier 532 are fabricated on asame integrated circuit (IC). In other embodiments, codec 530, currentreplicator 510, resistor 540, and amplifier 532 include an integratedsystem and may not be fabricated on a same IC, but may be a number ofseparate dies bonded or packaged together as a system on a chip (SoC) orcoupled to a common printed-circuit-board (PCB).

FIG. 6 illustrates a block diagram of a further embodiment microphonesystem 600 including a microphone 620, a current replicator 610, a codec630, and a current converter circuit 640. According to variousembodiments, microphone 620, current replicator 610, and codec 630function as described previously with reference to the other figures.Current converter circuit 640 is coupled between terminal 2 of currentreplicator 610 and input terminal INP of codec 630. In variousembodiments, current converter circuit may also be coupled to areference terminal GND as shown.

According to various embodiments, current converter circuit 640 mayconvert a replica current from current replicator 610 that isproportional to a current produced by microphone 620 into a voltagesignal. The converted voltage signal from current converter circuit 640may be supplied to the input INP to codec 630. Current converter circuit640 may include an impedance. In some specific embodiments, currentconverter circuit 640 may include resistors, capacitors, inductors, orany combination thereof. Current converter 640 may also include variousactive components, such as diode, amplifiers, and/or transistors. In afurther specific embodiment, current converter circuit 640 includes atransimpedance amplifier (TIA) having an input coupled to terminal 2 ofcurrent replicator 610 and an output coupled to input INP of codec 630.In such an embodiment, the TIA may have a reference node coupled to theground terminal GND. Further, current converter circuit 640 may includeany combination of active and passive components.

According to various embodiments, current converter circuit 640 may beused in any of the embodiments described herein. Further, all theembodiment components, including transducers, microphones, currentreplicators, current converter circuits, and codecs may be interchangedfreely between the embodiments described in the figures.

FIG. 7 illustrates a block diagram of an embodiment method of operation700 including steps 702 through 714. According to various embodiments,step 702 includes receiving power at an interface circuit on a powersupply terminal. Step 704 includes providing the received power to atransducer through a transducer terminal via the interface circuit. Step706 includes receiving a transduced signal from the transducer terminal.In various embodiments, the transduced signal is proportional to aphysical signal incident on the transducer. In a specific embodiment,the transduced signal is proportional to an acoustic signal received bya microphone that produces the transduced signal. Power may be suppliedto the transducer through a transducer terminal and a transduced signalmay be received from the transducer through the same transducerterminal.

According to further embodiments, step 708 includes providing a currentsignal proportional to the transduced signal to a current convertercircuit. Step 710 includes converting the current signal to a voltagesignal via the current converter circuit, and step 712 includesproviding the voltage signal to a codec. Finally, step 714 may includeproviding a reference supply to the codec. In various embodiments, thereference supply may be a ground connection.

According to an embodiment, an interface circuit includes a currentreplicator and a receiver. The current replicator includes a powerterminal coupled to a first reference node, an output terminalconfigured to output a signal proportional to a signal received from atransducer, and an interface terminal coupled to the transducer. Using asingle interface terminal, the current replicator may be configured toprovide power to the transducer and receive output signals from thetransducer. The receiver may include a first input terminal coupled tothe output terminal, a second input terminal coupled to a secondreference node, and a current converter circuit coupled to the firstinput terminal.

In various embodiments, the receiver further includes a codec configuredto encode analog signals and coupled to the current converter circuit.The current replicator and the receiver may be disposed on a sameintegrated circuit. The current replicator and the receiver may also bedisposed on two different integrated circuits. In some embodiments, thecurrent replicator and the receiver may include discrete components. Theinterface circuit may also include the transducer, and the transducermay be coupled to the interface terminal and the second reference node.The transducer may be implemented as a microphone. In such anembodiment, the microphone may be a microelectromechanical systems(MEMS) microphone. In some embodiments, the current converter circuit isfurther coupled to the second reference node.

In various embodiments, the current replicator includes a firsttransistor, a second transistor, and a differential amplifier. The firsttransistor may have a control terminal, a first conduction terminalcoupled to the power terminal, and a second conduction terminal coupledto the output terminal. The second transistor may have a controlterminal, a first conduction terminal coupled to the power terminal, anda second conduction terminal coupled to the interface terminal. In someembodiments, the control terminal of the second transistor is coupled tothe control terminal of the first transistor. The differential amplifiermay have an output coupled to the control terminals of the first andsecond transistors, a first input coupled to a reference voltage, and asecond input coupled to the second conduction terminal of the secondtransistor. In some embodiments, the first transistor, the secondtransistor, and the amplifier are configured to maintain a substantiallyconstant voltage at the second conduction terminal of the secondtransistor and replicate the current flowing from the first conductionterminal to the second conduction terminal of the second transistor inthe current flowing from the first conduction terminal to the secondconduction terminal of the first transistor.

According to an embodiment, a method of operating an interface circuitincludes providing power to a transducer on a first line, receiving atransduced signal from a transducer on the first line, and providing acurrent signal proportional to the transduced signal to a currentconverter circuit. In such an embodiment, the transduced signal isproportional to a physical signal received by the transducer. The methodof operating an interface circuit may further include converting thecurrent signal to a voltage signal at the current converter circuit,providing the voltage signal to a codec, and providing a referencesupply to the codec. In an embodiment, the method also includesmaintaining a voltage level on the first line at a constant voltagelevel.

According to an embodiment, a microphone system includes a microphone, acurrent replicator, a current converter circuit coupled between a secondterminal and a first reference node, and a codec coupled to the currentconverter circuit. In various embodiments, the microphone includes anoutput terminal and a reference terminal, the reference terminal may becoupled to the first reference node, and the microphone is configured toreceive power from the output terminal. Further, the current replicatormay include a first transistor with a conduction path coupled between afirst terminal and a second reference node, and a second transistor witha conduction path coupled between a second terminal and the secondreference node. The single output terminal may be coupled to the firstterminal.

In various embodiments, the current converter may include an impedancecoupled between the second terminal and the first reference node, andthe codec may be coupled to the second terminal. The impedance mayinclude resistive and/or capacitive elements. In other embodiments, thecurrent converter may include a transimpedance amplifier having an inputcoupled to the second terminal and an output coupled to the codec. Themicrophone may be implemented as a microelectromechanical systems (MEMS)microphone.

In various embodiments, the microphone and the codec may be disposed ontwo different integrated circuits. The current replicator may bedisposed on a same integrated circuit as the codec. In some embodiments,a control terminal of the first transistor may be coupled to a controlterminal of the second transistor. The current replicator may alsoinclude an amplifier having an output coupled to the control terminalsof the first and second transistors. An amplifier coupled to the currentconverter circuit may also be included in the codec.

Advantages of various embodiments described herein may include increasedtransducer voltage swing at a supply and sense interface terminal causedby a low voltage drop interface circuit.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A transducer system comprising: a transducer; anamplifier comprising a power supply terminal, a signal input terminalcoupled to the transducer, and a signal output terminal; and a currentreplicator coupled to the power supply terminal and not to the signaloutput terminal, the current replicator configured to receive a currentdraw signal from the power supply terminal, and generate a currentoutput signal based on the current draw signal, wherein the currentoutput signal is proportional to a physical signal received by thetransducer.
 2. The transducer system of claim 1, further comprising acurrent converter circuit coupled to the current replicator andconfigured to convert the current output signal to a voltage outputsignal.
 3. The transducer system of claim 2, wherein the currentconverter circuit consists of passive circuit components.
 4. Thetransducer system of claim 2, wherein the current converter circuitcomprises active circuit components.
 5. The transducer system of claim4, wherein the active circuit components comprise a transimpedanceamplifier.
 6. The transducer system of claim 1, wherein the currentreplicator comprises: a power terminal configured to be coupled to afirst reference node; an output terminal configured to output thecurrent output signal; an interface terminal configured to receive thecurrent draw signal from the power supply terminal; a first transistorhaving a control terminal, a first conduction terminal coupled to thepower terminal, and a second conduction terminal coupled to the outputterminal; a second transistor having a control terminal, a firstconduction terminal coupled to the power terminal, and a secondconduction terminal coupled to the interface terminal, wherein thecontrol terminal of the second transistor is coupled to the controlterminal of the first transistor; and a differential amplifier having anoutput coupled to the control terminals of the first and secondtransistors, a first input coupled to a reference voltage node, and asecond input coupled to the second conduction terminal of the secondtransistor.
 7. The transducer system of claim 6, wherein the firsttransistor, the second transistor, and the differential amplifier areconfigured to maintain a substantially constant voltage at the secondconduction terminal of the second transistor and replicate the currentflowing from the first conduction terminal to the second conductionterminal of the second transistor in the current flowing from the firstconduction terminal to the second conduction terminal of the firsttransistor.
 8. The transducer system of claim 1, wherein the transducercomprises a microphone.
 9. The transducer system of claim 8, wherein themicrophone comprises a microelectromechanical systems (MEMS) microphone.10. The transducer system of claim 9, further comprising a codec coupledto the current replicator and configured to receive the current outputsignal.
 11. The transducer system of claim 1, wherein the transducer andthe amplifier are disposed on a first integrated circuit die and thecurrent replicator is disposed on a second integrated circuit die, thefirst integrated circuit die being separate from the second integratedcircuit die.
 12. The transducer system of claim 1, wherein the signaloutput terminal and a reference supply terminal are coupled to a groundnode.
 13. The transducer system of claim 12, further comprising aresistive element having a first terminal and a second terminal, thesecond terminal directly connected to the ground node, wherein thereference supply terminal is directly connected to the ground node andthe signal output terminal is directly connected to the first terminalof the resistive element.
 14. A method of operating a transducer system,the method comprising: receiving a physical signal at a transducer;generating a transduced signal at the transducer based on the physicalsignal; supplying the transduced signal to a signal input terminal of anamplifier, the amplifier comprising the signal input terminal, a signaloutput terminal, and a power supply terminal; generating a current drawsignal at the power supply terminal; receiving the current draw signalat a current replicator, the current replicator not being coupled to thesignal output terminal; generating a current output signal based on thecurrent draw signal in the current replicator, wherein the currentoutput signal is proportional to the physical signal; and providing thecurrent output signal at an output of the current replicator.
 15. Themethod of claim 14, further comprising converting the current outputsignal to a voltage output signal using a current converter circuitcoupled to the output of the current replicator, the current convertercircuit comprising active circuit components.
 16. The method of claim14, wherein receiving the current draw signal at the current replicatorcomprises maintaining a substantially constant voltage at the powersupply terminal.
 17. The method of claim 16, wherein generating thecurrent output signal based on the current draw signal comprisesreplicating the current draw signal in the current output signal. 18.The method of claim 14, wherein receiving the physical signal at thetransducer comprises receiving an acoustic signal at a microphone. 19.The method of claim 14, further comprising providing the current outputsignal to a codec coupled to the output of the current replicator. 20.The method of claim 19, wherein providing the current output signal tothe codec comprises: converting the current output signal to a voltageoutput signal at a current converter circuit coupled to the output ofthe current replicator; and providing the voltage output signal from thecurrent converter circuit to the codec.
 21. A transducer systemcomprising: a first integrated circuit die comprising: a first interfacepin, a second interface pin coupled to a first reference voltage node, amicroelectromechanical systems (MEMS) transducer, and an amplifiercomprising a power supply terminal coupled to the first interface pin, asignal input terminal coupled to the MEMS transducer, and a signaloutput terminal coupled to the second interface pin; and a secondintegrated circuit die comprising: a current interface pin coupled tothe first interface pin, and a current mirror coupled to the currentinterface pin, the current mirror configured to receive a current drawsignal from the power supply terminal through the current interface pin,and supply a current output signal to an output node, wherein thecurrent output signal mirrors the current draw signal and isproportional to a physical signal received by the MEMS transducer. 22.The transducer system of claim 21, wherein the second integrated circuitdie further comprises a current converter circuit coupled to the outputnode and configured to convert the current output signal to a voltageoutput signal.
 23. The transducer system of claim 22, wherein thecurrent converter circuit comprises active circuit components.
 24. Thetransducer system of claim 23, wherein the active circuit componentscomprise a transimpedance amplifier.
 25. The transducer system of claim21, wherein the second integrated circuit die further comprises a codeccoupled to the output node.
 26. The transducer system of claim 21,wherein the current mirror comprises: a first transistor having acontrol terminal, a first conduction terminal coupled to a supplyvoltage node, and a second conduction terminal coupled to the outputnode; a second transistor having a control terminal, a first conductionterminal coupled to the supply voltage node, and a second conductionterminal coupled to the current interface pin, wherein the controlterminal of the second transistor is coupled to the control terminal ofthe first transistor; and a differential amplifier having an outputcoupled to the control terminals of the first and second transistors, afirst input coupled to a second reference voltage node, and a secondinput coupled to the second conduction terminal of the secondtransistor.
 27. The transducer system of claim 21, wherein the firstintegrated circuit die further comprises a third interface pin coupledto the first reference voltage node, wherein the second interface pin iscoupled to the first reference voltage node through a resistor; and theamplifier further comprises a reference supply terminal coupled to thethird interface pin.
 28. The transducer system of claim 21, wherein theMEMS transducer comprises a MEMS microphone.